Bagworms Can Ruin Your Summer

By on June 1, 2018

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Landscape & Garden Editor Kevin Johnson Shares Three Ways to Control This Perennial Pest

Years ago when I was just getting started in the green industry, I noticed a Blue Spruce standing dead on a property which we cared for. All that was left was what appeared to be small pine cones hanging from the limbs of the dead tree. Under closer examination we discovered that they were not Blue Spruce cones, but the cone-shaped bags of a pesky caterpillar called bagworms.

Bagworms, Thyridopteryx ephemeraeformis (Haworth), not to be confused with the Eastern Tent Caterpillar, are a type of caterpillar that feed on many evergreen shrubs, especially conifers such as arborvitae, juniper, pine, spruce, and many other evergreen species, including Leyland cypress. It also attacks certain deciduous trees such as black locust, honeylocust, and sycamore.

Bagworms are identified by the bag or case the caterpillar forms, which they hang from their host plant. The bag is made of silk and foliage from the host plant. The foliage is woven into the silk bag and helps disguise this clever caterpillar. As the caterpillar matures the bags will increase in size from 30-50 mm long.

The spread of the bagworm is slow since adult females are unable to fly and never leave the bag. She is worm-like lacking eyes, wings, legs, and a mouthpart. The male is black and moth-like with transparent wings.

Females lay 500-1000 eggs in each bag during the fall. Their ability to produce this many eggs can be deadly to the host plant. Eggs hatch in late spring or early summer. The larva crawl out of the bags and feed on the host plant for several weeks. Some plants can handle defoliation while others will die. The larva then pupate for roughly 4 weeks at which time the males crawl out of their bags and find females to continue their life cycle.

Keep in mind that bagworms are a perennial pest, year-round vigilance will help keep them from destroying your trees and shrubs. Late spring and early summer is a good time to get ahead of the game.
There are three means of controlling these perennial pests.

Mechanical

While I was supervising the ground department at Y.H.C., I had the crew remove bagworms by hand from a prized Blue Spruce in the presidential walkway. Pick off the bags during the winter and destroy them. This must be done before the eggs hatch in June. When too many plants are involved, to make hand picking practical, sprays are in order.

Biological

Bacillus thuringiensis, often called Bt, is a type of bacteria that only kills certain insects and does not affect humans or animals. Bt must be applied between mid-June and mid-July because it works well only on young bagworms. Commercially available under the following common brand names: Dipel, Thuricide, and others, Many of these brands are sold in local hardware stores and garden centers.

Chemical

If chemical control is absolutely necessary, a registered insecticide should provide control if applied thoroughly to all infested plant foliage after July 15. Check the label on the pesticide to be sure bagworm and the type of plant you wish to spray are listed. Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
If you notice your prized evergreen tree is suffering, at the same time there are peculiar cones hanging from the branches, you may have bagworms. ACLM

Kevin Johnson is the owner of Green Leaf Lawn and Ornamental, LLC, based in Blue Ridge. For more information about the devastating hemlock woolly adelgid and treatment options, Kevin can be reached toll free at 866. 883. 2420 or by email at growitgreen@etcmail.com.

Or check out his web site at www.wetreatlawns.com or visit www.hemlocks.org for more info.